Temperature variance nullification in an inrush current suppression circuit

ABSTRACT

The temperature dependence of an inrush current suppression circuit comprising a MOSFET having an input terminal coupled to a direct current input voltage can a transistor electrically coupled to the MOSFET can be reduced by matching the temperature coefficient of a transistor to a component electrically coupled to the transistor.

CLAIM OF PRIORITY

This application claims domestic priority under 35 U.S.C. §120 as aContinuation of prior U.S. patent application Ser. No. 12/368,273 nowU.S. Pat. No. 7,830,168, filed on Feb. 9, 2009, the entire contents ofwhich are hereby incorporated by reference as if fully set forth herein.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to power converter circuits andrelates more particularly to inrush current suppression circuitry for DCto DC and AC to DC converters.

BACKGROUND

The approaches described in this section are approaches that could bepursued, but are not necessarily approaches that have been previouslyconceived or pursued. Therefore, unless otherwise indicated, it shouldnot be assumed that any of the approaches described in this sectionqualify as prior art merely by virtue of their inclusion in thissection.

In electrical systems ranging in size from small portable electronicdevices to large industrial machinery it is common for differentcomponents in a system to operate at different voltage levels. In orderfor those different components to operate off of a common power supplysuch as a battery or generator, components operating at differentvoltages must be connected to the power source through a powerconverter, such as a direct current (DC) to DC converter or analternating current (AC) to DC converter. Power converters also improvethe interchangeability and scalability of components by eliminating theneed for every component to have its own, customized power source.

Inrush current suppression circuitry currently known in the art hasundesirable temperature variance, and depending on the applicationutilizing the power converter, the power converter might need to operateunder conditions ranging from below −55° C. to above 95° C. This widerange of operating temperatures greatly limits the extent to whichinrush current can be suppressed. For example, inrush currentsuppression circuitry known in the art for a typical DC-to-DC powerconverter operating at steady state currents of between 4 and 5 ampsmight be able to limit inrush current to 15 amps, but it is oftendesired to limit inrush current to much lower amounts, such as 8.5 amps.Achieving this combination of narrow current ranges and widely varyingoperating temperatures, however, is extremely difficult, if notimpossible, with the temperature-dependent inrush current suppressioncircuitry known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings.

FIG. 1 shows a block diagram of a power converter.

FIG. 2 a shows a graph of inrush current in the absence of inrushcurrent suppression circuitry.

FIG. 2 b shows a graph of inrush current in a system with inrush currentsuppression circuitry.

FIG. 3 shows a block diagram of a power converter implementing inrushcurrent suppression circuitry in an embodiment.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however,that the present invention may be practiced without these specificdetails. In other instances, well-known structures and devices are shownin block diagram form in order to avoid unnecessarily obscuring thepresent invention.

FIG. 1 is a schematic of a power converter 100 such as a DC-to-DC powerconverter. The power converter 100 can receive a differential inputvoltage at first inputs 110 a-b of a power bus from a power source suchas a battery or generator, and output a different differential voltageto a load at outputs 120 a-b. The converter contains various types ofconditioning circuitry 130 a-b for purposes such as raising or loweringthe voltage, eliminating noise, and implementing various safetyfeatures.

When the input voltage at the inputs 110 a-b is constant or nearconstant, the components of the conditioning circuitry 130 a-b operatein a steady state, and the input current of the power converter 100 willeither be constant or only slightly vary. When the power converter 100experiences a line transient with a high slew rate, such as when theoutput voltage of a power source connected to inputs 110 a-b suddenlyincreases, there is a time delay between the time the line transient isfirst received at the power converter 100 and the time the conditioningcircuitry 130 a-b returns to steady state operation.

The delay is caused by the time it takes capacitive elements in theconditioning circuitry 130 a-b to charge in response to the higher inputvoltage. In a capacitive element, because change in voltage (also calleddV/dt or slew rate) is proportional to change in current, the capacitiveelements will begin pulling large amounts of current from the powersource until the capacitor is charged, at which time a steady statecurrent is reached. During the time the capacitive elements arecharging, the input current will suddenly and temporarily spike beforereturning to a constant or near constant value. Many times, the currentduring such a spike will be 10 to 20 times greater than the steady statecurrent.

FIG. 2 a shows a graph of input current at the power converter 100during the time of a line transient. Prior to time T1, the input currentis constant at a value of A1. At time T1, the line transient occurs andthe incoming current spikes to A2 a by time T2 before returning to A3 attime T3. The spike in current to A2 a at time T2 might be damaging topower sources connected to the inputs 110 a-b or to devices connected tothe outputs 120 a-b of the power converter 100.

In order to suppress the spike in current, the power converter 100 canfurther comprise inrush current suppression circuitry 140 to prevent theinput current spike shown in FIG. 2 a. By including the inrush currentsuppression circuitry 140, the input current can be suppressed to alower level, such as A2 b as shown in FIG. 2 b.

Inrush current suppression circuitry currently known in the art hasundesirable temperature variance, and depending on the applicationutilizing the power converter, the power converter might need to operateunder conditions ranging from below −55° C. to above 95° C. This widerange of operating temperatures greatly increases the total circuittolerance and thus required additional window between the maximum steadystate current level and maximum allowable peak inrush current level. Forexample, inrush current suppression circuitry known in the art for atypical DC-to-DC power converter operating at a steady state current of5 amps might have ±4 A of variation in the current limit threshold andthus must be set to a nominal current limit point of at least 9 amps(which would allow it to vary to as much as 13 A) to avoid trippingduring steady state operation, but it is often desired to limit inrushcurrent to much lower amounts, such as 8.5 amps. Achieving thiscombination of narrow current ranges and widely varying operatings,however, is extremely difficult, if not impossible, with thetemperature-dependent inrush current suppression circuitry known in theart. Therefore, there exists in the art a need for inrush currentsuppression circuitry that is less affected by changes in temperature.

FIG. 3 shows a power converter device containing inrush currentsuppression circuitry 340, in one embodiment. The power converter canreceive a differential input voltage at first inputs 310 a-b from apower source such as a battery or generator, and output a differentdifferential voltage to a load at outputs 320 a-b. For example, theinput voltage might be 270V, and the output voltage might be 12V. Theconverter contains various types of conditioning circuitry 330 a-b suchas for raising or lowering the voltage, eliminating noise, andimplementing various safety features.

The power converter of FIG. 3 further comprises inrush currentsuppression circuitry 340. In an embodiment, inrush current suppressioncircuitry 340 includes a metal-oxide-semiconductor field-effecttransistor (MOSFET) 341 with a drain 341 d, a gate 341 g, and a source341 s. The drain 341 d of the MOSFET 341 can be coupled throughconditioning circuitry 330 a to a power source such as a generator, andthe gate 341 g of the MOSFET 341 can be connected through a resistor 345to a bias voltage V_(BIAS), such as 12V for example. When thegate-to-source voltage of the MOSFET 341 drops below a threshold level(referred to hereinafter as the plateau voltage), the MOSFET 341 willenter the linear region of operation wherein the drain-to-sourceimpedance will begin increasing, and the MOSFET 341 can be used as avariable resistor.

Once the MOSFET 341 enters its linear region, the drain-to-sourceimpedance of the MOSFET 341 can be used to create a voltage drop overthe inrush current suppression circuitry 340, i.e. between 330 a and 330b. Due to the voltage drop, the components comprising conditioningcircuitry 330 b will not draw as much current from the power sourceconnected to inputs 310 a-b.

By controlling the gate-to-source voltage of the MOSFET 341, the voltagedrop over the MOSFET 341 can be adjusted as a function of the amount ofinrush current at inputs 310 a-b, such that once the gate-to-sourcevoltage is below the plateau voltage, increased amounts of in rushcurrent will cause a higher drain-to-source impedance and hence a higherdrain-to-source voltage drop.

To control the gate-to-source voltage of the MOSFET 341, the activeinrush current suppression circuitry 340 further comprises a transistor342, such as a bipolar junction transistor (BJT), with a collector 342c, an emitter 342 e, and a base 342 b. The collector 342 c of thetransistor 342 is electrically coupled to the gate 341 g of the MOSFET341. The transistor 342 is a non-linear device and can draw little or nocurrent into the collector 342 c when the base-to-emitter voltage(V_(BE)) of the transistor 342 is below a V_(BE) threshold voltage butcan begin drawing a large amount of current into the collector 342 cwhen V_(BE) exceeds that V_(BE) threshold voltage. The V_(BE) thresholdvoltage can be a low voltage relative to the input voltage. For example,the input voltage might be 270V, and the V_(BE) threshold voltage mightbe 0.7V at room temperature.

The base 342 b of the transistor 342 can be electrically coupled to theemitter 342 e of the transistor 342 by a resistive element such as aresistor 343 (also referred to as R_(SENSE) 343) and another component,such as a Schottky diode 344, connected in series. Thus, in theembodiment of FIG. 3, V_(BE) will be the voltage drop over R_(SENSE) 343and the Schottky diode 344.

The value of R_(SENSE) 343 can be chosen to be relatively low comparedto the resistance caused by the MOSFET 341 when it is operating in itslinear region, such that under a line transient condition, the majorityof the voltage drop caused by the inrush current suppression circuitry340 will be caused by the MOSFET 341 and not R_(SENSE) 343.

The resistance of R_(SENSE) 343 can additionally be chosen such that insteady state operation the voltage drop over R_(SENSE) 343 and theSchottky diode 344 will be below the V_(BE) threshold value. Under aline transient condition, however, the voltage drop across R_(SENSE) 343and the Schottky diode 344 can increase to above the V_(BE) thresholdvoltage, causing the transistor 342 to start conducting current into thebase 342 b. When the transistor 342 conducts current into the base 342b, an amplified amount of current will be drawn into the collector 342c. The current drawn into the collector 342 c will have an amount ofgain, such as a factor of 10 to 200 depending on the parameters of thetransistor 342, relative to the current drawn into the base 342 b.

The current drawn into the collector 342 c comes primarily from thegate-to-source parasitic capacitance of the MOSFET 341, discharging thegate 341 and thus reducing the gate-to-source voltage. Reducing thegate-to-source voltage to below the plateau voltage can causes theMOSFET 341 to enter its linear region, thus causing the drain-to-sourceimpedance to increase. Therefore, the transistor 342 can be used tocreate a feedback loop, such that the drain-to-source impedance of theMOSFET 341 is a function of the amount of current through R_(SENSE) 343.

Transistors currently known in the art frequently have significanttemperature coefficients associated with their base-to-emitter thresholdvoltages, such that the V_(BE) threshold voltage will typically vary −2mV/° C. Therefore, the V_(BE) threshold voltage for a device operatingover a temperature range of −45° C. to +95° C. might vary by as much as0.3V, which is significant considering that V_(BE) threshold voltagestend to be relatively small, such as 0.7V.

In order to offset this temperature variance in the transistor 342,embodiments of the present approach include coupling another component,such as a Schottky diode 344, in series to R_(SENSE) 343, such that theV_(BE) of the transistor 342 is the voltage drop over both the Schottkydiode 344 and R_(SENSE) 343. The Schottky diode 344 can be chosen suchthat its temperature coefficient is similar to the transistor's 342temperature coefficient. For example, if the temperature coefficient ofthe transistor 342 is approximately −2 mV/° C., then a Schottky diodewith a temperature coefficient of approximately −2 mV/° C. might also bechosen.

The Schottky diode 344 can be coupled to the base 342 b of thetransistor 342 and through a resistor R_(BASE) 346 to V_(BIAS). Theresistance of R_(BASE) 346 and the parameters of the Schottky diode 344can be chosen such that the Schottky diode 344 will have a low forwardvoltage drop. For example, at room temperature the forward voltage dropover the Schottky diode 344 might be 0.25V. If the room temperatureV_(BE) threshold voltage for the transistor 342 is 0.7V and the roomtemperature forward drop over the Schottky diode 344 is 0.25V, then aresistance for R_(SENSE) can be selected such that the voltage drop overR_(SENSE) 343 will be greater than 0.45V at the current level where itis desired that the inrush current suppression circuitry 340 beginssuppressing the inrush current, also referred to as the current limitpoint. When the current through R_(SENSE) 343 causes a voltage dropacross R_(SENSE) 343 of greater than 0.45V then V_(BE), then thecombined voltage drop across the Schottky diode 344 and R_(SENSE) 343will exceed the V_(BE) threshold voltage of 0.7V that causes the MOSFET341 to enter its linear region, as described above.

At an operating temperature higher than room temperature, such as 95°C., the V_(BE) threshold voltage of the transistor 340 might decreasefrom 0.7V to 0.55V, for example. If the Schottky diode 344 has a similartemperature coefficient, then at 95° C. its forward voltage drop mightdecrease by the same or a similar amount, i.e. from 0.25V to 0.1V. Ifthe forward voltage drop of the Schottky diode 344 is 0.1V and theV_(BE) threshold voltage is 0.55V, then a voltage drop of greater than0.45V across R_(SENSE) 343 is once again the voltage drop that causesthe MOSFET 341 to enter its linear region. As resistance does not varysignificantly with temperature, the current limit point that causes thevoltage drop across R_(SENSE) 343 to exceed 0.45V will also not changesignificantly.

At an operating temperature significantly lower than room temperature,such as −45° C., the V_(BE) threshold voltage of the transistor 340might increase from 0.7V to 0.85V. If the Schottky diode 344 has asimilar temperature coefficient, then at −45° C. its forward voltagedrop might increase by the same or a similar amount, i.e. from 0.25V to0.4V. If the forward voltage drop of the Schottky diode 344 is 0.4V andthe V_(BE) threshold voltage is 0.55V, then once again a voltage drop ofgreater than 0.45V across R_(SENSE) 343 is necessary for the MOSFET 341to enter its linear region. As in the instance where the operatingtemperature is significantly above room temperature, at operatingtemperatures significantly below room temperature, the voltage dropacross R_(SENSE) 343 required for V_(BE) to exceed the V_(BE) thresholdvoltage remains the same or similar, meaning the current limit pointalso remains the same or similar.

Although the previous example, presents a scenario where a Schottkydiode 344 is used to keep the current limit point and the associatedvoltage required across R_(SENSE) 343 constant, it will be readilyapparent to one skilled in the art that a Schottky diode 344 or othercomponent with a temperature coefficient different than the temperaturecoefficient of the transistor 342 might also be used to reduce variationof the current limit point as a function of temperature, as opposed tokeeping the current limit point constant.

A power converter embodying the techniques herein can be configured tohave a steady state current of between 4 amps and 5 amps when coupled toa power source having a voltage of approximately 270V. In response to aline transient with a slew rate of greater than 20V/μs, inrush currentsuppression circuitry can limit inrush current to less than 9 ampsacross a temperature range from −55° C. to 95° C.

In this description certain process steps are set forth in a particularorder, and alphabetic and alphanumeric labels may be used to identifycertain steps. Unless specifically stated in the description,embodiments of the invention are not necessarily limited to anyparticular order of carrying out such steps. In particular, the labelsare used merely for convenient identification of steps, and are notintended to specify or require a particular order of carrying out suchsteps.

In the foregoing specification, embodiments of the invention have beendescribed with reference to numerous specific details that may vary fromimplementation to implementation. Thus, the sole and exclusive indicatorof what is the invention, and is intended by the applicants to be theinvention, is the set of claims that issue from this application, in thespecific form in which such claims issue, including any subsequentcorrection. Any definitions expressly set forth herein for termscontained in such claims shall govern the meaning of such terms as usedin the claims. Hence, no limitation, element, property, feature,advantage or attribute that is not expressly recited in a claim shouldlimit the scope of such claim in any way. The specification and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense.

1. An apparatus comprising: a metal-oxide semiconductor field-effecttransistor (MOSFET) having an input terminal directly or indirectlyelectrically coupled to a direct current input voltage; a transistorelectrically coupled to the MOSFET and configured to cause an impedanceof the MOSFET to increase in response to a sensed voltage exceeding athreshold voltage, the sensed voltage to increase in response to a rapidincrease in input current, wherein the threshold voltage is a lowvoltage relative to the direct current input voltage; a componentelectrically coupled to the transistor, and configured to have a voltagedrop that varies as a function of temperature; wherein the sensedvoltage comprises the voltage drop of the component and a voltage dropover a resistive element, the voltage drop of the resistive element tobe a function of a current of the direct current input voltage.
 2. Theapparatus of claim 1, wherein the component is a Schottky diode.
 3. Theapparatus of claim 1, wherein the component is coupled either directlyor indirectly to a base of the transistor and an emitter of thetransistor.
 4. The apparatus of claim 1, wherein the component isconnected in series to a resistor, and the resistor and component couplea base of the transistor to an emitter of the transistor.
 5. Theapparatus of claim 1, wherein the sensed voltage comprises the voltagedrop of the component.
 6. The apparatus of claim 1, wherein abase-to-emitter threshold voltage of the transistor has a firsttemperature coefficient and the component has a second temperaturecoefficient, and wherein the first temperature coefficient and secondtemperature coefficient are either both positive or both negative.
 7. Acircuit comprising: a MOSFET having an input terminal directly orindirectly coupled to a direct current input voltage; a transistorcoupled to the MOSFET and configured to cause an impedance of the MOSFETto increase in response to a base-to-emitter voltage of the transistorexceeding a threshold voltage, wherein the threshold voltage decreasesas temperature increases and is a low voltage relative to the directcurrent input voltage; a component electrically coupled either directlyor indirectly to the base of the transistor and to the emitter of thetransistor, wherein the base-to-emitter voltage comprises a voltage dropacross the component, the voltage drop across the component to vary as afunction of temperature; wherein the sensed voltage comprises thevoltage drop of the component and a voltage drop over a resistiveelement, the voltage drop of the resistive element to be a function of acurrent of the direct current input voltage.
 8. The circuit of claim 7,wherein the component is a Schottky diode.
 9. The circuit of claim 7,wherein the component is connected in series to a resistor, and thebase-to-emitter voltage comprises a voltage drop across the resistor.10. The circuit of claim 7, the threshold voltage of the transistor hasa first temperature coefficient and the component has a secondtemperature coefficient, and wherein the first temperature coefficientand second temperature coefficient are either both positive or bothnegative.
 11. A circuit comprising: a MOSFET having an input terminaldirectly or indirectly coupled to a direct current input voltage; atransistor coupled to the MOSFET, and configured to cause an impedanceof the MOSFET to increase in response to a base-to-emitter voltage ofthe transistor exceeding a threshold voltage, wherein the thresholdvoltage increases as temperature decreases and is a low voltage relativeto the direct current input voltage; a component electrically coupledeither directly or indirectly to the base of the transistor and to theemitter of the transistor, wherein the base-to-emitter voltage comprisesa voltage drop across the component, the voltage drop across thecomponent to increase as temperature decreases; wherein the sensedvoltage comprises the voltage drop of the component and a voltage dropover a resistive element, the voltage drop of the resistive element tobe a function of a current of the direct current input voltage.
 12. Thecircuit of claim 11, wherein the component is a Schottky diode.
 13. Thecircuit of claim 11, wherein the component is connected in series to aresistor, and the base-to-emitter voltage comprises a voltage dropacross the resistor.
 14. The circuit of claim 11, the threshold voltageof the transistor has a first temperature coefficient and the componenthas a second temperature coefficient, and wherein the first temperaturecoefficient and second temperature coefficient are either both positiveor both negative.
 15. A power converter circuit comprising: inputsconfigured to connect to a power source, wherein a current of greaterthan 4 amps is drawn from the power source during steady stateoperation; an inrush current suppression circuit configured to limitinrush current to less than 9 amps in response to a line transientcondition at the power source having a slew rate of 20 V/μs, the inrushcurrent suppression circuitry to limit inrush current to less than 9amps when operating at −45° C. and 95° C.; wherein the inrush currentsuppression circuit comprises a metal-oxide semiconductor field-effecttransistor (MOSFET) having an input terminal directly or indirectlyelectrically coupled to a direct current input voltage of the powersource; a transistor electrically coupled to the MOSFET and configuredto cause an impedance of the MOSFET to increase in response to a sensedvoltage exceeding a threshold voltage, the sensed voltage to increase inresponse to a rapid increase in the direct current input voltage; acomponent electrically coupled to the transistor, the component to havea voltage drop that varies as a function of temperature; wherein thethreshold voltage is a low voltage relative to the direct current inputvoltage.
 16. The power converter circuit of claim 15, wherein thecomponent is a Schottky diode.
 17. The power converter circuit of claim15, wherein the component is connected in series to a resistor, and thebase-to-emitter voltage comprises a voltage drop across the resistor.